Casing joints, liners, tubing, and other oilfield tubulars are often used in drilling, completing, and producing a well. Casing joints, for example, may be emplaced in a wellbore to stabilize a formation, to protect a formation against elevated wellbore pressures (e.g., wellbore pressures that exceed a formation pressure), and the like. Casing joints may be coupled in an end-to-end manner by threaded connections, welded connections, and other connections known in the art. The connections may be designed so as to form a seal between an interior of the coupled casing joints and an annular space formed between exterior walls of the casing joints and walls of the wellbore. The seal may be, for example, an elastomeric seal (e.g., an o-ring seal), a metal-to-metal seal formed proximate the connection, or similar seals known in the art.
Metal-to-metal seals for threaded connections have been designed in many forms. One example of a metal-to-metal seal is disclosed in U.S. Pat. No. 5,462,315 issued to Klementich. That patent is incorporated herein by reference in its entirety. A connection disclosed by Klementich is shown in FIG. 4. The connection includes a two-step external thread 403A and 403B formed on a tubular section 101. Tubular section 102 has a two-step internal thread 402A and 402B that corresponds to the two-step external thread 403A and 403B. Klementich discloses forming corresponding center shoulder seal structures 405 and 406 between the steps on the two-step external thread 403A and 403B and the two-step internal thread 402A and 402B, respectively. At a selected make-up, the corresponding center shoulder seal structures 405 and 406 form a metal-to-metal center shoulder seal, which also acts as a positive-stop torque shoulder. As used herein, “make-up” refers to threading a pin member (external thread) and a box member (internal thread) together. “Selected make-up” refers to threading the pin member and the box member together with a desired amount of torque, or based on a relative position (axial or circumferential) of the pin member with the box member. “Back-off” or “backing-off” refers to the unthreading of corresponding pin members and box members.
One type of connection used for casing joints and tubing is the coupled connection. A prior art coupled connection is shown in FIG. 1. In FIG. 1, a coupled connection includes a coupling 103 with two sets of internal threads 105A and 105B, which correspond to external threads 106A and 106B on tubular sections 101 and 102, respectively. Tubular sections 101 and 102 have the external thread 106A at one end and the external thread 106B at the other end. In some instances, external threads 106A and 106B may have different properties (e.g. thread pitch and thread form). In the coupled connection shown in FIG. 1, the pin nose surfaces 108A and 108B are designed to come into contact at a central plane 110 in the coupling 103. A metal-to-metal seal may be formed by the contact between the pin nose surfaces 108A and 108B.
U.S. Pat. Nos. 4,762,344 and 6,817,633 issued to Perkins and Brill, respectively, disclose coupled connections in which pin noses 108A and 108B contact each other. Those patents are incorporated herein by reference in their entireties. Brill discloses that having contact between pin nose surfaces 108A and 108B serves as a torque shoulder for making-up the coupled connection. Perkins discloses having contact between the pin nose surfaces 108A and 108B such that a seal may by formed when the connection is selectively made-up. For a typical coupled connection, the tubular sections 101 and 102 have two pin members each, while the coupling 103 has two box members corresponding to the pin members.
In FIGS. 5 and 6, two alternative sealing arrangements for coupled connections that are disclosed by Perkins are shown. In FIG. 5, pin nose surfaces 208A and 208B are formed at corresponding angles that are non-perpendicular (inclined or declined) to the axis (not shown) of the tubular sections 101 and 102. At a selected make-up, the pin nose surfaces 208A and 208B contact each other to form a metal-to-metal seal while functioning as a positive-stop torque shoulder. Also, the coupling 103 includes an inwardly projecting annular ridge 140 that, at the selected make-up, engages the outer diameters of the pin noses at an axial position that overlaps pin nose surfaces 208A and 208B. In FIG. 6, the coupling 103 has external threads 106A and 106B formed thereon and the tubular sections 101 and 102 have internal threads 105A and 105B formed thereon, respectively. The tubular sections 101 and 102 have two pairs of corresponding sealing surfaces on their respective ends. Seal surfaces 601A and 601B are formed at corresponding angles that are non-perpendicular to the axis of the tubular sections 101 and 102, which act both as a metal-to-metal seal and a positive-stop torque shoulder. The corresponding angles of seal surfaces 601A and 601B also radially trap the tubular sections 101 and 102 together during make-up. The other pair of sealing surfaces 602A and 602B is at a relatively shallow angle with respect to the axis of the tubular sections 101 and 102.
Referring again to FIG. 1, a typical method for making-up a coupled connection is to make-up the coupling 103 to the end of the tubular section 102 that will be pointing upward toward the surface while in the wellbore. This leaves a box member (internal thread 105A) facing upward. The pin member (external thread 106A) may then be stabbed into the coupling 103 to be made-up. When the pin noses 108A and 108B are designed to contact each other, the making-up of the second tubular section 101 with the coupling 103 causes rubbing to occur between the pin noses 108A and 108B because the tubular sections 101 and 102 are rotated relative to each other. The rubbing can cause the pin noses 108A and 108B to gall, which can prevent a reliable seal from being formed.
To prevent damage to the seal between the pin noses 108A and 108B, the above-mentioned U.S. Pat. No. 4,762,344 issued to Perkins discloses having external threads 106A and 106B that have the same “hand” when facing opposite of each other. In other words, one of the set of threads is manufactured as a left-handed thread (i.e. rotate left to tighten), while the other set of threads is manufactured as a traditional right-handed thread (i.e. rotate right to tighten). This allows for the coupling to be rotated relative to the tubular sections 101 and 102 to make-up the connection, while the tubular sections 101 and 102 do not rotate relative to each other. To be able to securely make-up the connection, Perkins discloses having different thread pitches on the two sets of threads. For example, internal thread 105B and external thread 106B may have a thread pitch of about 3 threads per inch (“TPI”), while internal thread 105A and external thread 106A may have a thread pitch of about 12 TPI.
To use such a coupled connection, external thread 106B may be first partially made-up to a selected position with the internal thread 105B, which may set by having marks (not shown) on the outer diameters of the tubular section 102 and the coupling 103. The other external thread 106A may then be partially made-up to a selected position with the internal thread 105A. The selected position may be such that the pin noses 108A and 108B are close to contacting each other or lightly contacting each other. To finish making-up the connection, both tubular sections 101 and 102 may be restrained from rotating relative to each other while the coupling 103 is rotated. The different thread pitches cause the pin noses 108A and 108B to be brought together axially without relative rotation between the tubular sections 101 and 102. Torque continues to be applied to the coupling 103 until the selected make-up is achieved, which is typically determined by the amount of torque applied to the coupling.
Besides avoiding galling of the seal surface, there are other circumstances where little or no relative rotation of tubular sections is desirable during make-up. For example, long sections of spoolable tubing may be used in various applications, including down hole operations. Some tubular sections may be over 3,000 feet in length, while the wellbore may be 2 or more miles in measured depth. In making-up those connections, rotation of the already deployed tubular section and the still spooled tubular section requires twisting the tubular sections. A coupled connection that does not require rotation of the tubular sections may be used in that circumstance. Temporary pipelines on and around drilling locations may also be suitable for such a coupled connection.
In general, casing and tubing are exposed to greater internal pressure (burst) than external pressure (collapse). Accordingly, fluid and/or gas leaking from the inside of the tubular to the outside due to internal pressure is more common. What is still needed is a pin nose seal that is externally supported to provide an improved seal against internal pressure.